Field of the Invention
The present invention relates to inspection apparatus and methods usable, for example, to perform defect detection in the manufacture of devices by lithographic techniques. The invention further relates to an illumination system for use in such inspection apparatus and to methods of manufacturing devices using lithographic techniques. The invention yet further relates to computer program products for use in implementing such methods.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Careful control and adjustment of the process is required to avoid defects such as broken lines or bridged lines. Metrology tools are used to check for defects in the applied pattern. Defect metrology is one of the most important metrology of semiconductor industry, as it deals directly with the semiconductor fab yield. Often defects are related to certain ‘hot spots’ on a substrate, so that metrology efforts can be concentrated on those areas. The dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques at visible wavelengths. Small features include for example those formed by multiple patterning processes, and/or pitch-multiplication. While scanning electron microscopy (SEM)_is able to resolve these modern product structures directly, SEM is much more time consuming than optical measurements.
The inventor has considered whether the techniques of coherent diffraction imaging (CDI), combined with radiation of wavelength comparable with the product structures of interest, might be applied to defect detection on modern device structures. CDI is also known as lensless imaging, because there is no need for physical lenses or mirrors to focus an image of an object. The desired image is calculated synthetically from a captured light field. Various techniques for CDI are described in the PhD thesis describing lensless imaging at EUV wavelengths is “High-Resolution Extreme Ultraviolet Microscopy” by M. W. Zürch, Springer Theses, DOI 10.1007/978-3-319-12388-2_1. A particular type of CDI is ptychography, described for example in published patent application US 2010241396 and U.S. Pat. Nos. 7,792,246, 8,908,910, 8,917,393, 8,942,449, 9,029,745 of the company Phase Focus Limited and the University of Sheffield. D. Claus et al provide an introduction to ptychography in a paper “Ptychography: a novel phase retrieval technique, advantages and its application” Proc. SPIE 8001, International Conference on Applications of Optics and Photonics, 800109 (Jul. 26, 2011); doi:10.1117/12.893512. In ptychography, phase information is retrieved from a plurality of captured images with an illumination field that is moved slightly between successive captures. Overlap between the illumination fields allows reconstruction of phase information and 3-D images. Other types of CDI can be considered also.
Another example of CDI is known as ankylography, which offers the potential to determine properties of a 3-D structure from a single capture. In order to do this, an image of a radiation field is obtained, that has been diffracted by an object, for example a microstructure made by lithography. Literature describing ankylography at EUV wavelengths includes: the paper “Designing and using prior data in Ankylography: Recovering a 3D object from a single diffraction intensity pattern” E. Osherovich et al http://arxiv.org/abs/1203.4757 and the PhD thesis by E. Osherovich “Numerical methods for phase retrieval”, Technion, Israel—Computer Science Department—Ph.D. Thesis PHD-2012-04-2012). Other approaches are described in a Letter by K S Raines et al “Ankylography: Three-Dimensional Structure Determination from a Single View”, published in Nature 463, 214-217 (14 Jan. 2010), doi:10.1038/nature08705 and in a related presentation by Jianwei (John) Miao, KITP Conference on X-ray Science in the 21st Century, UCSB, 2-6 Aug. 2010 (available at http://online.kitp.ucsb.edu/online/atomixrays-c10/miao/).